As is well known, in modern surgery there are various prostheses for the treatment of many types of vascular diseases. In particular, vascular prostheses—used in cardiac surgery, vascular surgery and in invasive cardiology—can be ideally divided into extravascular prostheses, intended to be implanted on the outer surface of the vessels, and intravascular prostheses, better known as endoprostheses, which, in general, are implantable within vessel or vascular structures.
For example, in the field of cardiac surgery, and in particular in pediatric cardiac surgery, it is well known the so-called Ross Procedure, which uses the autologous pulmonary artery (so-called pulmonary autograft) to replace the aortic root, with a replanting of the coronary arteries. In the following, reference will be made also to the technique of so-called Freestanding Ross Root Replacement, to indicate that surgical procedure wherein the pulmonary valve and the pulmonary artery trunk are used as ideal substitutes for pathological aortic valve replacement in block with the aortic root. (Sievers et al A multicentre evaluation of the autograft procedure for young patients undergoing aortic valve replacement: update on the German Ross Registry † Eur J Cardiothorac Surg. 2015 Feb. 9. pii: ezv001).
Currently, the Ross Procedure is considered one of the most appropriate procedures for the treatment of aortic valve pathology in children and young adults, as it brings significant advantages in both surgical results and patient management as well as in long-term results.
The transposition of the pulmonary artery trunk in the aortic position, in fact, avoids the known drawbacks of anticoagulation—which usually occurs in conjunction with the use of mechanical prostheses—and provides excellent results in terms of valve hemodynamics.
However, the major drawback of this procedure is represented, in the long term, by the dilatation of the pulmonary artery autograft in the aortic root position. This phenomenon leads to complications, such as the appearance of an aortic regurgitation due to excessive tension on the commissure, the progressive increase of the diameter of the pulmonary artery autograft and, exceptionally, acute dissection of the pulmonary trunk in the aortic position. These complications often imply the need for a new surgical intervention, which is carried out a few years after the first procedure.
To overcome the above drawbacks, a modified procedure has been developed, which involves the use of an extravascular prosthesis, consisting of a synthetic, non-resorbable reinforcement of the pulmonary artery autograft. This extravascular prosthesis is normally made of a synthetic material such as Dacron, tetrapolyethylene or Polytetrafluoroethylene (PTFE).
However, in this type of surgery the use of synthetic materials involves long-term clinical and biological disadvantages. A first important drawback is represented by the inability of such materials to adjust to the biological tissue during the structural development of the vessel. For example, the tetrapolyethylene has a stiffness approximately 24 times greater than the stiffness of the native aorta. Therefore, the ability of the prosthesis to conform to the vessel to which it is applied is very low. This causes the loss of elastomechanical properties and of the “Windkessef” function of the neoaortic root, with retrograde effect on the aortic valve, leading ultimately to its incompetence.
A further disadvantage of these solutions according to the known art is represented by the strong inflammatory reaction caused by the aforementioned synthetic materials, which would lead to a damage to the vitality of the pulmonary artery autograft, interfering with the normal arterialization process.
Conversely, the treatment of aneurysmatic disease of the aorta, peripheral and cerebral vessels majorly relies in the use of intravascular prostheses or vascular stents, i.e. mesh-like cylindrical structures intended to be introduced in the vascular lumen so that they can expand until their diameter is equal to that of the lumen.
Some of the stents presently available are made of an auxetic material, i.e. a material having a negative Poisson ratio. Expanded Polytetrafluoroethylene (Expanded PTFE), also known by the common trade name Goretex®, is an example of auxetic material. When subjected to a tensile stress, the fibers of such material “open up” structurally and the material expands in the direction transverse to the stress. On the contrary, if this material were subject to compression, it would “close” structurally.
One of the problems of the abovementioned intravascular prosthesis is that often they are not able to adapt their conformation or compliance over time to the patient's tissue, especially to vessels of large lumen which tend to grow.
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